This invention relates to a receiving device for receiving a magnetic field in connection with a goods monitoring system.
In SE-PS 8404691-1 a system for detecting the presence of indicating devices is described which have the form, for example, of a narrow and thin, but relatively long strip of a highly permeable material, such as being commercially available under the name Permalloy. The presence of such strips is detected by means of coils, which emit and receive magnetic fields, and by means of devices for the detection of signals received.
When such a strip is exposed to a relatively weak external magnetic field, it assumes in its longitudinal direction a magnetic flux density, the magnitude of which can be twenty thousand times higher than in the external magnetic field. The presence of such strips is detected by a detector device, in which variations in voltage induced in a receiver coil are detected.
According to said patent, two coils are caused to emit a magnetic alterating field of high frequency, where a first one of the coils emits an alternating field with a frequency, which is different from the frequency of the field emitted by the second one of the coils, and where at least one difference and/or sum frequency n . f.sub.1 +m . f.sub.2 arising by intermodulation by the indicating device, where n and m are positive or negative integers, is caused to be received by one or several coil. According to the patent, a third magnetic alternating field of low frequency is caused to be emitted in said zone which has a frequency substantially lower than said frequencies f.sub.1,f.sub.2, where the low-frequency field is caused to have a field intensity sufficiently high for, independently of the fields with the frequencies f.sub.1 and f.sub.2, putting the indicating device into saturated condition in step with the frequency of the low frequency field. A field re-emitted from the indicated device is caused to be detected by means of a detector device as an intermodulation, occurring periodically with the frequency of the low-frequency field, between the frequencies f.sub.1 and f.sub.2.
One problem with known theft-prevention systems is that the orientation of the strip in the space is of great importance for whether or not the presence of the strip in the investigation zone will be detected and, thus, whether of not alarm will be sounded. This problem normally has been solved in such a way, that emitting and receiving coils were positioned in different places relative to the investigation zone.
Another problem is that a simple receiving coil is relatively sensitive to interferences of various kind, originating from other electric devices installed in shop premises.